The realism of a sampled instrument depends on a number of factors, but key among them is sample size. If you are limited in the amount of storage your samples can fill, then you must either a) use samples that leave out a lot of information and thus loose a lot of the subtlety, or b) use samples that have a lot of information but are very short. Option b works well for drums, for instance, but not for instruments that have a sustained sound, such as a violin or even a piano. Often people have solved this problem by looping - finding two zero crossings within the sample of a single note, and then causing the sampler to play up to the first zero crossing, 'til the end of the second zero crossing, and then repeat the segment between the first and second zero crossing until it's time to stop playing the note. This is a reasonable solution, but it means that you lose the ability to have the sound of a sampled instrument evolve over time the way the original would.

If you can use extremely large samples, though, you can have a lot of sample information over a lot of time. Gigastudio (and its predecessor gigasampler) allow you to do this; it's not unusual for samples to be in the multi-gigabyte range. For comparison, perhaps a decade ago EMU released the Audio Production Studio card, which allowed you to have samples that totalled 32 megabytes.

The typical solution to this has been to keep all sample data in memory, so that you can just send it right to the soundcard as soon as you get the keypress. However, recall that I said that gigastudio could use samples on the order of several gigabytes. Most of us (as of the time of this writing) don't have several gigabytes of memory.

Nemesys solved this problem by keeping the beginning of each sample in memory, and the rest on disk. When they receive a keypress, the play the sound out of memory and simultaneously start sending it from the disk to the soundcard. This means they get rapid response but aren't so limited by memory constraints.

Why is it only for Windows?

Nemesys states that the reason their software works only on Windows is that they have to create specialized soundcard drivers to get the data from disk to the soundcard without going through memory. They had to choose a platform to target, so they chose the one with the most users.

They argue that, if you are wedded to a Mac or some other system, it's still cheaper to buy a cheap windows box with a good soundcard (you can get one relatively cheap since it only has to provide output, not handle input) and Gigastudio, than to buy a comparable hardware sampler. They also argue that Windows tends to be fairly stable if you only ever run one application on it.

Is there anything else like this?

Yes, there are at least two other products out there that do the same thing: HALion from Steinberg and Kontakt by Native Instruments. Both have some advantages over the current version of Gigastudio: Kontakt, at least, is available for the Macintosh, and both allow you to plug in a more standard palette of software fx - Gigastudio allows you to use only NFX effects, and the number of those available is pretty small.

Nemesys states they are working on a new version of Gigastudio (the current version is 2.5), which they expect to have out by the end of the year.

Is it any good?

It kicks ass. HALion and Kontakt are, I understand, giving it a run for its money, but I wasn't too impressed with Kontakt (though admittedly I tried it for only about an hour) and haven't tried HALion yet.